In order to explain the background of the invention, reference will be particularly made to FIG. 1. The reference numeral 1 designates a GaAs substrate. The numeral 2 designates a Ga.sub.1-x Al.sub.x As lower cladding layer, the numeral 3 designates a Ga.sub.1-y Al.sub.y As active layer, and the numeral 4 designates a Ga.sub.1-x Al.sub.x As upper cladding layer. The numeral 5 designates a GaAs contact layer, the numeral 6 designates a substrate electrode, the numeral 7 designates a stripe electrode, and the numeral 10 designates an end surface of the laser resonator. The bottom two layers, substrate 1 and the lower cladding layer 2, and the top two layers, upper cladding layer 4 and the contact layer 5, have mutually opposite conductivity types. That is, when the former two, 1 and 2, are n types, the latter two, 4 and 5 are p types, and vice versa. There is a relation that x&gt;y among the aluminium composition proportions x and y.
The operation of the device is described as follows:
When it is presumed that the substrate 1 is n type, the lower cladding layer 2 is n type, the upper cladding layer 4 and the contact layer 5 are p types, and the active layer 3 is of either of the two types. The three layers of the lower cladding layer 2, the active layer 3, and the upper cladding layer 4, where the aluminium composition proportion x is larger than y, constitute a double heterostructure, and under this structure the confinement of injected carriers and light are achieved, and it leads to a laser oscillation. The generated laser light is emitted from the light emitting portion 3a at the resonator end surface 10 which is the result of the cleavage.
This prior art semiconductor laser does not have a window structure at the light emitting end surface. In other words, the band gap of the active layer 3 in the neighborhood of the light emitting end surface 10 is equal to the band gap of the active layer inside the resonator, and therefore, most of the light generated at the active layer 3 is absorbed at the laser emitting end surface 3b of the active layer 3 where a lot of surface energy levels exist. This increases the likelihood that the laser emitting end surface 3b will be destroyed by heat produced by the absorption of light when the light output is increased. Aiming at overcoming the disadvantages described above, there are several attempts to develop a method of producing a semiconductor laser having a window structure. However, these methods have disadvantages in that they are difficult manufacturing methods and may produce astigmatism of the laser beam.
As another prior art of a semiconductor laser, there is an article, "Monolithic high-power dual-wavelength GaAlAs laser array" by M. Wada et al, Appl. Phys. Lett. 43(10) Nov. 15, 1983, p903 to p905. In this article it is described that the production of ridges or terraces on a substrate makes the growth velocities of portions on the ridges or terraces different, thereby resulting in variations of the composition proportion in AlAs of the portions.
Another prior art of a semiconductor laser is reported in an article, "A high-power, single-mode laser with twin-ridge-substrate structure" by M. Wada et al, Appl. Phys. Lett. 42(10), May 15, 1983 p853 to 854. In this article it is described that production of an active layer on a ridge of a substrate by a liquid phase growth method results in a low growth velocity, thereby enabling one to obtain a thin active layer easily.
Other prior art of a semiconductor laser is reported in an article, "Visible GaAlAs V-channeled substrate inner stripe laser with stabilized mode using p-GaAs substrate" by S. Yamamoto et al, Appl. Phys. Lett. 40(5), Mar. 1, 1982, p372 to p374. This article shows a transverse mode controlled semiconductor laser to which the present invention is applied to obtain a third embodiment.